The present invention relates to peelably sealed packaging, and more particularly to peelable films useful to incorporate in peelably sealed packaging.
Peelably sealed, xe2x80x9ceasy-openxe2x80x9d packaging is used in a variety of food and non-food packaging applications to provide packaging that is easy for the consumer to open without having to tear the packaging. Peelably sealed packaging may take several forms. For example, a flexible film lidding or lid stock material may be peelably adhered to a rigid tray, tub, or container. Alternatively, the easy-open packaging may incorporate a peelable seal between a combination of one or more of flexible films, webs, substrates, or supports. Examples of this alternative include thermoforming and vacuum skin packaging methods known in the art. For example, the lower web or support (e.g., xe2x80x9cformed webxe2x80x9d) may be heated and deep-drawn to form a receptacle for the item to be packaged. Once the item is placed on the support, the upper web (e.g., xe2x80x9cnon-formed webxe2x80x9d) is drawn over the item and peelably sealed to the peripheral edges of the support. The seal may be formed using heated sealing bars, platens, or frames to apply heat and pressure to the top and bottom webs in the seal area.
Other types of easy-open packaging may be formed using vertical-form-fill-seal (VFFS) or horizontal-form-fill-seal (HFFS) methods known in the art to form a pouch or bag containing the packaged product. In such alternatives, the peelable film or web may in effect be sealed to itself (i.e., the same type of film material may form both the top and bottom web of the packaging).
The layer of the peelable film that primarily facilitates the easy-open, peelable seal is known as the xe2x80x9cpeelable layerxe2x80x9d or xe2x80x9cseparation layer.xe2x80x9d If the film is a mono-layer film, then the film itself may be considered the peelable layer. If the peelable layer is an outer layer of a multi-layer film, then the peelable layer may be a sealant layer (e.g., heat-seal layer) of the film.
A peelable layer may also be an internal layer of a multi-layer film. In such an arrangement, one or more layers of a film may be hand peeled away (i.e., delaminated) from the remaining layers of the film, which remain attached to the substrate. This design may be used, for example, to peel away oxygen-impermeable layers of a film from the oxygen-permeable layers of the film at a desired time after the original construction of the packaging. Such a design is described, for example, in U.S. Pat. No. 5,919,547 issued Jul. 6, 1997 to Kocher, which is incorporated herein in its entirety by reference.
To open an easy-open packaging, the consumer simply grasps a portion of one web and pulls or xe2x80x9cpeelsxe2x80x9d it away from a second web or supportxe2x80x94thereby causing the peelable seal to xe2x80x9cfail.xe2x80x9d The type of failure depends on the design and location of the peelable layer. With an xe2x80x9cinterfacial peelxe2x80x9d or xe2x80x9csurface peel,xe2x80x9d the peelable layer may be located as an outer sealant layer of the top film, so that the top film separates from the substrate to which it adheres at the interface between the peelable layer of the top film and the substrate. Alternatively, with a xe2x80x9cdelamination peel,xe2x80x9d the peelable layer may be located as an internal layer of the top film, so that the top film separates between the peelable layer and another layer of the top film (i.e., interlayer separation).
The peelable seal failure mechanism may be further classified as: 1) a xe2x80x9ccohesive failure,xe2x80x9d in which the molecular attractive forces holding together the peelable layer composition are overcome, or 2) an xe2x80x9cadhesive failure,xe2x80x9d in which the interfacial forces (e.g., valence forces or interlocking action or both) holding two surfaces together are overcome, or 3) a combination of both. Regardless of the type of peelable seal failure, it is desirable that the failure not cause xe2x80x9cangel hairxe2x80x9d or strings of resin to form or remain between the peeled layer and the substrate upon peeling.
The peelable seal should be sufficiently strong to withstand the expected abuse during the packaging operation, distribution, and storage. If the seal bond strength is too weak, then the package may open prematurely. However, the peelable seal must also provide a sufficiently low bond strength to allow the end-user to easily peel open the package by hand. If too high of a peel force is needed to open the peelably sealed packaging, then the peelable film may be considered xe2x80x9clocked downxe2x80x9dxe2x80x94that is, too hard to open easily by hand.
If the peelable layer of the film is an outer, sealant layer of the film, then it is desirable that the peelable layer have a relatively low heat seal initiation temperature in order to allow the equipment used to form the heat seals to run at relatively fast speeds. Further, it is also desirable that a packager be able to increase the bond strength of the peelable seal merely by increasing the heat seal temperature at which the peelable seal is made. This is because the packager may desire to form a less easily peeled seal for some applications (e.g., liquid packaging) than others (e.g., dry cereal packaging). However, it is also desirable that the peelable seal strength gradually increase as the sealing temperature increases, so that the sealing temperature does not have to be monitored so closely by the packager. Otherwise, a relatively small drift or variation in the sealing temperature that may be associated with the sealing equipment may cause a large variation in the peelable seal strength.
The bottom web or substrate of an easy-open package is often an ionomer-based surface or an LLDPE-based surface, depending on the packaging application and the desired performance attributes. However, existing peelable films that form an acceptable peelable seal at a given temperature with an ionomer-based surface generally do not tend to form an acceptable peelable seal at the same given temperature with an LLDPE-based surface. Conversely, peelable films that form an acceptable peelable seal at a given temperature with an LLDPE-based surface generally do not tend to form an acceptable peelable seal at the same given temperature with an ionomer-based surface. As a result, packagers that use both ionomer-surfaced substrates and LLDPE-surfaced substrates in providing easy-open packaging must currently purchase at least two separate types of top film or lid stock depending on the composition of the substrate. Further, to meet this demand, peelable film suppliers must be able to provide at least two types of compatible top film or lid stock to the packagers. This increases the amount of peelable film inventory that the easy-open packager or peelable film supplier must carryxe2x80x94and also increases the complexity and cost of the manufacture of easy-open packaging.
The present invention addresses one or more of the aforementioned problems. A peelably sealed package includes a web having a peelable layer and a substrate having a surface layer. The peelable layer includes a blend of: i) from about 3 to about 15 weight parts polybutylene, ii) from about 40 to about 75 weight parts ionomer, and iii) from about 20 to about 55 weight parts ethylene/unsaturated ester copolymer. The unsaturated ester may be a vinyl ester of aliphatic carboxylic acid, where the vinyl ester has from 4 to 12 carbon atoms, or an alkyl ester of acrylic or methacrylic acid, where the alkyl ester has from 4 to 12 carbon atoms. The surface layer of the substrate includes a polyethylene having a density of from about 0.915 to about 0.93 g/cc, an ionomer, or a mixture of these polymers. The peelable layer of the web and the surface layer of the substrate are peelably sealed to each other in one or more selected areas.
In another aspect, a thermoplastic film useful as a peelable film has at least one layer comprising a blend of from about 3 to about 15 parts polybutylene, from about 40 to about 75 parts ionomer, and from about 20 to about 55 weight parts ethylene/unsaturated ester copolymer. The unsaturated ester is a vinyl ester of aliphatic carboxylic acid, where the vinyl ester has from 4 to 12 carbon atoms, or an alkyl ester of acrylic or methacrylic acid, where the alkyl ester has from 4 to 12 carbon atoms.
The inventive peelable film is useful in forming peelably sealed packaging. A packager may use the same peelable film with substrates having either a polyethylene contact surface or an ionomer contact surfacexe2x80x94without having to significantly adjust or change the heat sealing temperature of the sealing equipment. This allows the packager to reduce the inventory of peelable film stock because the same peelable film may be used to form effective peelable seals with either type of substrate. Further, because the heat sealing temperature does not need to be adjusted, the use of the inventive peelable film enhances the flexibility and speed of changeover in packaging configurations between the ionomer and polyethylene surfaced substrates. Since the peel strength of the peelable seal increases as the heat seal temperature increases, the packager may increase the peel strength merely by increasing the heat seal temperature. Further, because the peel strength gradually increases with the increase in heat seal temperature, the packager does not have to monitor the sealing temperature so closely to avoid a relatively small drift or variation in the sealing temperature from causing a large change in the peel strength of the peelable seal.
These and other objects, advantages, and features of the invention will be more readily understood and appreciated by reference to the detailed description of the invention and the drawings.